Solenoid operated directional control valves have long been available; however, they have been the on/off-type valves which when operated electrically shift to a fully open or fully closed position. In more recent times, electro-hydraulic servo valves have been developed which accurately control the velocity, acceleration, and position of actuators by an electrical signal controlling a hydraulic output. These valves can be used to meter flow to and from hydraulic actuators or to control a variable displacement pump. Servo valves of this nature are either single-stage or double-stage with the latter being prevalent where the pressures and flow rates are significant. In a two-stage valve, the main control spool is actuated by a double acting actuator which is supplied by a pilot valve which is in turn controlled by some form of electro-mechanical transducer such as a solenoid or torque motor. What the pilot stage does is take a low level mechanical signal, amplify it and with the amplified signal control the main control valve spool. A variety of different types of amplifiers in the pilot stage have been used such as spool-type valves; jet-pipe type, single-flapper type and double-flapper type. Also, these types of valves include a feedback function whereby the position of the main control spool provides a signal to the pilot stage so that any error in the main stage can be corrected. This feedback function provides the fine metering and accuracy factor achieved in current generation servo valves.